In its popular acceptation cursing is often confounded, especially in the phrase "cursing and swearing", with the use of profane and insulting language; in canon law it sometimes signifies the ban of excommunication pronounced by the Church. In its more common Biblical sense it means the opposite of blessing (cf. Num., xxiii, 27), and is generally either a threat of the Divine wrath, or its actual visitation, or its prophetic announcement, though occasionally it is a mere petition that calamity may be visited by God on persons or things in requital for wrongdoing. Thus among many other instances we find God cursing the serpent (Gen., iii, 14), the earth (Gen., iii, 17), and Cain (Gen., iv, 11). Similarly Noe curses Chanaan (Gen., ix, 25); Josue, him who should build the city of Jericho (Jos., vi, 26-27); and in various books of the Old Testament there are long lists of curses against transgressors of the Law (cf. Lev., xxvi, 14-25; Deut., xxvii, 15, etc.). So, too, in the New Testament, Christ curses the barren fig-tree (Mark, xi, 14), pronounces his denunciation of woe against the incredulous cities (Matt., xi, 21), against the rich, the worldling, the scribes and the Pharisees, and foretells the awful malediction that is to come upon the damned (Matt., xxv, 41). The word curse is also applied to the victim of expiation for sin (Gal., iii, 13), to sins temporal and eternal (Gen., ii, 17; Matt., xxv, 41).

In moral theology, to curse is to call down evil upon God or creatures, rational or irrational, living or dead. St. Thomas treats of it under the name maledictio, and says that imprecation may be made either efficaciously and by way of command, as when made by God, or inefficaciously and as a mere expression of desire. From the fact that we find many instances of curses made by God and his representatives, the Church and the Prophets, it is seen that the act of cursing is not necessarily sinful in itself; like other moral acts it takes its sinful character from the object, the end, and the circumstances. Thus it is always a sin, and the greatest of sins, to curse God, for to do so involves both the irreverence of blasphemy and the malice of hatred of the Divinity. It is likewise blasphemy, and consequently a grievous sin against the Second Commandment, to curse creatures of any kind precisely because they are the work of God. If, however, the imprecation be directed towards irrational creatures not on account of their relation to God, but simply as they are in themselves, the guilt is no greater than that which attaches to vain and idle words, except where grave scandal is given, or the evil wished to the irrational creature cannot be separated from serious loss to a rational creature, as would be the case were one to wish the death of another's horse, or the destruction of his house by fire, for such wishes involved serious violation of charity.

Curses which imply rebellion against Divine Providence, or denial of His goodness or other attributes, such as curses of the weather, the winds, the world, the Christian Faith, are not generally grievous sins, because the full content and implication of such expressions is seldom realized by those who use them. The common imprecations against animate or inanimate objects which cause vexation or pain, those against enterprises which fail of success, so, too, the imprecations that spring from impatience, little outbreaks of anger over petty annoyances, and those spoken lightly, inconsiderately, under sudden impulse or in joke, are, as a rule, only venial sins -- the evil being slight and not seriously desired. To call down moral evil upon a rational creature is always illicit, and the same holds good of physical evil, unless it be desired not as evil, but only in so far as it is good, for example, as a punishment for misdeeds, or a means to amendment, or an obstacle to commission of sin; for in such cases the principal intention, as St. Thomas says, is directed per se towards what is good. When, however, evil is wished another precisely because it is evil and with malice prepense, there is always sin, the gravity of which varies with the seriousness of the evil; if it be of considerable magnitude, the sin will be grievous, if of trifling character, the sin will be venial. It is to be noted that merely verbal curses, even without any desire of fulfilment, become grievous sins when uttered against and in the presence of those who are invested with special claims to reverence. A child, therefore, would sin grievously who should curse father, mother, or grandfather, or those who hold the place of parents in his regard, provided he does so to their very face, even though he does this merely with the lips and not with the heart. Such an act is a serious violation of the virtue of piety. Between other degrees of kindred verbal curses are forbidden only under pain of venial sin. To curse the devil is not of itself a sin; to curse the dead is not ordinarily a grievous sin, because no serious injury is done them, but to curse the saints or holy things, as the sacraments, is generally blasphemy, as their relation to God is generally perceived.